Keep a check build whose key is unchanged instead of building it again

A fresh check asked when only the trunk moved rebuilt every image, though
the reuse key said nothing it is built from changed: a controller check
took about 12 minutes instead of 5.5. The check now keeps a build kept
under its key when a verdict of the build seat names that key and every
digest, names it as the build's own check so a walk takes it, and the
cleanup keeps it while any check names it (novox/hq issue 703).

internal/builder/reuse_test.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"net/http"
 	"net/http/httptest"
+	"reflect"
 	"slices"
 	"strconv"
 	"strings"
@@ -625,3 +626,122 @@ func TestABuildTwoChecksShareGoesWithTheLastOfThem(t *testing.T) {
 		t.Fatal("the build stayed after the last check naming it went")
 	}
 }
+
+// aCheckAgain is a later check (build-2) of aPull building the module from a tree, with the builds the build seat's
+// verdicts name for it (novox/hq issue 703).
+func aCheckAgain(t *testing.T, scratch *aScratch, tree string, leave []NamedBuild) (CheckBuilt, *recorded) {
+	t.Helper()
+	r, run, workspace := aModuleAt(t, tree)
+	spec := CheckSpec{ID: "build-2", Owner: aPull.Owner, Repo: aPull.Repo, Number: aPull.Number,
+		Builds: []CheckBuildSpec{{Module: "meshboard", Repository: "https://forge.invalid/meshboard.git", Path: "mod",
+			Ref: "main"}}}
+	built := buildForTheMerge(t.Context(), run, spec, CheckBuilding{Publish: r, Scratch: scratch, Named: leave},
+		workspace, "", "", CheckVerdict{Head: "fedcbafedcba", Trunk: "654321654321"}, GitCredential{}, nil, logging(nil))
+	if len(built) != 1 || built[0].NotBuilt != "" || built[0].Key == "" {
+		t.Fatalf("the second check did not answer the module for its merge: %+v", built)
+	}
+	return built[0], r
+}
+
+// A check asked again when only the trunk moved, whose key is unchanged, builds nothing: it keeps the build an
+// earlier verdict names and names it by key and digest, so the walk can take it (novox/hq issue 703).
+func TestASecondCheckWhoseKeyIsUnchangedBuildsNothingAndNamesTheEarlierBuild(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	first := aCheckBuilt(t, scratch, "tree-one")
+
+	again, r := aCheckAgain(t, scratch, "tree-one", named(first))
+	if dockerBuilt(r) || len(r.images) > 0 || len(r.archives) > 0 {
+		t.Fatalf("the second check built again: ran %v, published %v %v", r.ran, r.images, r.archives)
+	}
+	if again.Key != first.Key || again.Kept != "build-1" || !reflect.DeepEqual(again.Made, first.Made) {
+		t.Fatalf("the second check names %+v, not the first check's build %+v", again, first)
+	}
+	// The walk takes it by what the second verdict names.
+	leave := named(again)
+	leave[0].Check = again.Kept
+	w, run, workspace := aModuleAt(t, "tree-one")
+	got, err := BuildReusing(t.Context(), run, w, Reuse{Scratch: scratch, Pulls: []PullRef{aPull}, Named: leave},
+		"https://forge.invalid/meshboard.git", "mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil || got.Reused == nil || dockerBuilt(w) {
+		t.Fatalf("the walk did not take the kept build: %v, why %q", err, got.Why)
+	}
+}
+
+// A check whose key changed builds, as before.
+func TestASecondCheckWhoseKeyChangedBuilds(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	first := aCheckBuilt(t, scratch, "tree-one")
+
+	again, r := aCheckAgain(t, scratch, "tree-two", named(first))
+	if !dockerBuilt(r) || again.Kept != "" || again.Key == first.Key {
+		t.Fatalf("a check whose key changed did not build: %+v", again)
+	}
+}
+
+// A scratch record no verdict names is never kept: the store takes writes from anything on its network, so a
+// check builds rather than name a planted record (novox/hq issue 703, ADR 0350 §2).
+func TestASecondCheckKeepsNoRecordAVerdictDoesNotName(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	first := aCheckBuilt(t, scratch, "tree-one")
+	for name, leave := range map[string][]NamedBuild{
+		"no verdict": nil,
+		"another digest": func() []NamedBuild {
+			n := named(first)
+			n[0].Digests["server"] = "sha256:" + strings.Repeat("e", 64)
+			return n
+		}(),
+		"another key": func() []NamedBuild { n := named(first); n[0].Key = "r1:" + strings.Repeat("f", 64); return n }(),
+	} {
+		t.Run(name, func(t *testing.T) {
+			again, r := aCheckAgain(t, scratch, "tree-one", leave)
+			if !dockerBuilt(r) || again.Kept != "" {
+				t.Fatalf("a record no verdict names was kept: %+v", again)
+			}
+		})
+	}
+}
+
+// A build a later check kept stays when the check that built it goes: its artifacts and its record, which the later
+// check's verdict names (novox/hq issue 703, ADR 0350 §5).
+func TestABuildALaterCheckKeptStaysWhenTheCheckThatBuiltItGoes(t *testing.T) {
+	m := &aTaggingRegistry{aRegistryOfRepositories: newRegistryOfRepositories(), tags: map[string]map[string]string{}}
+	server := httptest.NewServer(m.serve())
+	defer server.Close()
+	address := strings.TrimPrefix(server.URL, "http://")
+	registry := Registry{Address: address}
+	ctx := t.Context()
+	now := time.Date(2026, 10, 14, 9, 0, 0, 0, time.UTC)
+
+	body := `{"mediaType":"` + mediaManifestOCI + `","layers":[]}`
+	digest := digestOf([]byte(body))
+	m.manifests["scratch/meshboard/server@"+digest] = []byte(body)
+	key := "r1:" + strings.Repeat("1", 64)
+	b := CheckBuild{Key: key, Module: "meshboard", Pull: aPull, Check: "build-1", At: now,
+		Made: []catalogue.Built{{Name: "server", Kind: "image", Reference: address + "/scratch/meshboard/server@" + digest}}}
+	if err := registry.Keep(ctx, aPull, "build-1", now, []CheckBuild{b}); err != nil {
+		t.Fatal(err)
+	}
+	// Another pull request's check keeps it: its own document names the build, which stays the first check's.
+	other := PullRef{Owner: "novox", Repo: "meshboard", Number: 8}
+	if err := registry.Keep(ctx, other, "build-2", now, []CheckBuild{b}); err != nil {
+		t.Fatal(err)
+	}
+	held := func() bool { _, ok := m.manifests["scratch/meshboard/server@"+digest]; return ok }
+	found := func() bool { _, ok, _ := registry.Find(ctx, "meshboard", key); return ok }
+
+	if n, err := registry.Forget(ctx, aPull); err != nil || n != 1 {
+		t.Fatalf("forgot %d (%v)", n, err)
+	}
+	if !held() || !found() {
+		t.Fatalf("the build a later check names went with the check that built it: held %v, record %v", held(), found())
+	}
+	if n, err := registry.Forget(ctx, other); err != nil || n != 1 {
+		t.Fatalf("forgot %d (%v)", n, err)
+	}
+	if held() || found() {
+		t.Fatalf("the build stayed after the last check naming it went: held %v, record %v", held(), found())
+	}
+}